The proposed study will examine urinary sodium excretion induced by psychological stress and its diurnal pattern as a novel biological mechanism that may underlie an abnormal diurnal pattern of blood pressure (BP). BP follows a diurnal rhythm, for which it is highest during the daytime, and falls to its lowest level during the nighttime (i.e. BP dipping). The diurnal pattern of BP over 24 hours can be assessed using ambulatory BP monitoring (ABPM). An abnormal diurnal pattern of BP on ABPM, defined by reduced BP dipping or elevated nighttime BP, is associated with an increased risk of cardiovascular disease (CVD) events. Psychological stress is associated with an abnormal diurnal pattern of BP. However, the underlying mechanisms remain unknown. An exposure to an acute stressor using a mental stress task normally increases urinary sodium excretion. However, there is substantial between-person variability in the degree of stress- induced sodium excretion, such that some individuals have sodium retention after stress provocation. It has been hypothesized that when urinary sodium excretion does not occur during the daytime, urinary sodium excretion is shifted toward the nighttime, which is accompanied by an increase in nighttime BP. Therefore, individuals with lower stress-induced sodium excretion who experience psychological stress during the daytime may have a shift of urinary sodium excretion from the daytime to nighttime periods due to an inability to excrete sodium during the daytime. This altered diurnal pattern of urinary sodium excretion may be associated with a smaller decline in BP from the daytime to nighttime, leading to reduced BP dipping. The study will be conducted both in the laboratory setting and in the naturalistic environment with a multi-ethnic sample of 211 adult community participants from upper Manhattan. In the laboratory, urinary sodium excretion in response to mental stress tasks will be examined. In the naturalistic environment, measurement of the diurnal pattern of sodium excretion, ABPM, wrist actigraphy, and ecological momentary assessments of perceived stress will be performed. We hypothesize that lower stress-induced sodium excretion rate in the laboratory will be associated with lower daytime-to-nighttime ratio of urinary sodium excretion rate in the naturalistic environment; and lower daytime-to-nighttime ratio of urinary sodium excretion rate will be associated with reduced BP dipping. We will also examine whether the association between urinary sodium excretion after provoked stress and the diurnal pattern of sodium excretion is modified by higher ecological momentary levels of stress, experienced during the daytime. The study is highly innovative as it will test not previously proposed theoretical linkages among urinary excretion in response to stress provoked in the laboratory, ecological stress, and the diurnal patterns of sodium excretion and BP in the naturalistic environment. The study has high clinical significance for improving the understanding of the role of psychological stress in human hypertension, and has the potential for reducing the CVD risk associated with an abnormal diurnal pattern of BP.